A conventional paddy field is of a structure as shown in the schematic plan view of FIG. 31. That is, water of a river 1 is introduced through a channel 21 to a trunk watering channel 30 and supplied to paddy fields 4. The paddy fields 4 are divided into a plurality of farming plots 42 by balks 41. Farm roads 40 run in the longitudinal direction of the paddy fields 4, and open watering channels 31 are formed along the edges of the farm roads 40 and along the farming plots 42. The upstream side of each watering channel 31 is connected to the trunk watering channel 30. The wall of each watering channel 31 locating on the farming plot 42 side is provided with distribution ports 32. The irrigation water in the watering channel 31 is supplied through the distribution ports 32 to the farming plots 42 respectively, as shown in the perspective view of FIG. 32.
Meanwhile, an open drainage channel 33 is formed along the space between adjacent paddy fields 4, i.e. on the other sides across the paddy fields 4 from the watering channels 31. Water level setting devices 34 are installed at drain ports of the drainage channels 33 facing the respective farming plots 42, so that fixed flooded water levels may be maintained in the farming plots 42 respectively by adjusting the height of each water level setting device 34. The drainage channel 33 is connected on the downstream side to a trunk drainage channel 35, and the surplus irrigation water or rainwater drained into the trunk drainage channel 35 is adapted to be returned through a channel 22 to the river 1.
In case of paddy fields having such a structure, the irrigation water of each watering channel 31 flows through the distribution ports 32 to the farming plots 42 successively from the upstream side to the downstream side. The irrigation water supplied to each farming plot 42 is maintained at a fixed level by the water level setting device 34, and surplus irrigation water is drained into the drainage channel 33. Since irrigation water is supplied to the farming plots 42 successively from the upstream side toward the downstream side, supply of irrigation water tends to be poorer toward the downstream farming plots 42. Particularly, during the drought season, irrigation water is not substantially supplied to the downstream farming plots 42, causing reduction in the yield of rice plant due to insufficient growth.
Further, the conventional paddy fields where the channels are of open type involve a problem in that the farm cannot be utilized effectively, since the spaces for farming plots 42 and farm roads 40 are reduced by the widths occupied by the watering channels 31 and the drainage channel 33. Further, earth, sand, etc. are likely to fall into the open-top channels to deposit there, so that they must be removed frequently, requiring intricate care of the channels.
Meanwhile, as the number of farming families with side jobs increases with urbanization of agricultural villages, there are increasing demands on automation and energy saving in the farming work and the like, in turn, on labor saving in irrigation control of paddy fields, fertilization control, etc. due to reduction and limitation of the farming time.
A first objective of the present invention is to provide a paddy field water level regulating device which can supply irrigation water stably throughout paddy fields and can automatically maintain fixed flooded water levels in the paddy fields.
A second objective of the present invention is to provide a paddy field water level regulating device which enjoys excellent installability and can be provided inexpensively.
A third objective of the present invention is to provide a paddy field water level regulating device which can easily cope with changing crop production in some farming plots.